The present embodiments relate to non-contact transmission of optical signals.
Electronic devices may include one fixed device component (stator) and one device component (rotor) able to be rotated around the stator, such as computer tomographs, for example. Data may be transmitted between the stator and rotor. Optical transmission methods provide high bandwidth, excellent electromagnetic compatibility and low interference. However non-contact transmission of optical signals between rotor and stator presents the developer with an enormous challenge, as the pertinent patent literature shows.
A number of methods have been described for direct lateral coupling of optical signals into a ring-shaped optical waveguide. For example, German application DE 2846526 discloses a computer tomograph in which a fixed annular optical waveguide is irradiated by a light source emitting the optical signals to be transmitted which is attached to a unit able to be rotated around the patient with x-ray tubes and detector. The light coupled laterally into the optical waveguide is directed using internal reflections to a light receiver (detector) and evaluated there. A direct coupling in of optical signals into an annular optical waveguide can for example also be taken from the German publications DE 2732806 and DE 10256634 A1. The disadvantage of direct coupling in of signals is primarily the associated heavy signal attenuation.
An improvement in the transmission quality can be obtained by optical waveguides acting as optical amplifiers and being provided for this purpose with a material able to be excited into fluorescence. The creation of fluorescent light in the optical waveguide is excited by the incidence of lateral optical signals which are directed by internal reflections to a detector device and evaluated there. The disadvantage is that the fluorescent light occurs through spontaneous emission so that the modulation bandwidth for the optical signals to be transmitted is greatly restricted by the lifetime of the fluorescence excitation states.
A wider modulation bandwidth for the optical signal transmission can be achieved by an optical amplifier, as is described in European patent EP 0908030 B1. This discloses an apparatus for receiving optical signals which comprises an optical waveguide bent into the shape of a ring, into which the optical signals are able to be coupled from the side. The optical waveguide is provided with a material of which the arrangement of electrons can be put into a population inversion by energetic excitation and which emits light with an emission wavelength by stimulated emission. If optical signals are coupled into the population-inverted optical waveguide, light is created by stimulated emission in the optical waveguide, which is directed by internal reflections to a detector device and can be evaluated there.
Although the modulation bandwidth and power for the optical signal transmission can be increased by this apparatus compared to an optical waveguide provided with fluorescing material, the primary problem arising is that of a strong spontaneous emission, which results in light continuously escaping from the optical waveguide. Another complicating factor is that light created by the spontaneous emission exhibits large amplitude fluctuations, with a characteristic time scale for the amplitude fluctuations depending on the amplification and able to lie in the range of the symbol length (period for transmission of a logical value). For this reason the spontaneous emission is viewed as a source of noise with significant amplitude, by which a sensitive optical signal transmission can be disturbed.